Are replacement windows ever worth it? or What's wrong with my math?
Kitep
Energy savings: $9.25/yr
Payback time: 15.6 years
Is this typical?
The $144 is just the cost of the window, and does not include installation
costs (I'm not against installing it myself).
BTW, turns out finding the price for triple or quadruple glazed windows over
the internet is harder than I thought. I couldn't find a single one. The
above is for an Anderson Silver Line 8500 double-hung window, double glaze,
low-e.
----
Now for the math. Feel free to change channels :)
R = 1 / u
loss per degree day = A * (1 degree) * (24 hours/day) / (1/u) = 24 u A
Note: I'll use DD for number of degree-days/year
loss per year = 24 u A (DD)
Note: I'll use C for cost per BTU
cost per year = 24 u A (DD) C
savings per year = 24 A (DD) C (u_old - u_new)
The window mentioned above is 28" x 54" = 2.333' x 4.5' = 10.5 ft^2
(I'm not married to that window, but it was one I could find both price and
u-factor for)
degree days = 4928 (heating) + 1135 (cooling) = 6163
cost per BTU (natural gas) = ($1.5184/ccf) * (1 ccf / 102000 BTU) = $14.886
x 10^-6 / BTU
(I'm going to cheat, and not write out the units)
savings per year = 24 A (DD) C (u_old - u_new)
= 24 (10.5) (6163) (14.886 x 10^-6) (u_old - u_new)
= 23.119 (u_old - u_new)
u_new = .35 for above window
u_old = .75 (I'm guessing on this. Looks like a single pane, aluminum clad.
But there's also a storm window, so shouldn't it be half this?)
savings per year = 23.119 (u_old - u_new) = 23.119 (.75 - .35) = $9.25/year
Payback time = $144 / ($9.25/year) = 15.6 years
Even if I could get a super duper window (u=.20) for the same price, that
brings it to $12.72/year, or 11.32 year payback time.
What am I missing? Or have I just fallen for the hype about replacement
windows?
Eeyore
Kitep wrote:
> Window cost: $144
> Energy savings: $9.25/yr
> Payback time: 15.6 years
>
> Is this typical?
I believe it is.
The fastest payback for any energy use reduction method is insulation.
Loft/attic insulation pays back in a couple of years for example and blown in
cavity wall insulation is up there somewhere too.
Graham
Kitep
"Eeyore" <rabbitsfriend...@hotmail.com> wrote in message
news:48274FE2...@hotmail.com...
> The fastest payback for any energy use reduction method is insulation.
> Loft/attic insulation pays back in a couple of years for example and blown
> in
> cavity wall insulation is up there somewhere too.
More attic insulation was going to be next calculation. But after being
disappointed with the windows calc, I didn't have the heart. Plus I thought
maybe my math was off.
David Williams
-> Energy savings: $9.25/yr
-> Payback time: 15.6 years
-> Is this typical?
-> The $144 is just the cost of the window, and does not include installation
-> costs (I'm not against installing it myself).
You also haven't included the interest that you might have got by
investing the $144 instead of buying windows with it. Suppose you could
get 5% interest, so that would be $7.20 per year. You're forgoing that
amount in order to save $9.25, so you're actually only $2.05 ahead
each year. So it will take about 70 years to repay your $144
investment, assuming that nothing changes in that time.
There's probably something more useful you could do with the money.
dow
Eeyore
Kitep wrote:
> "Eeyore" <rabbitsfriend...@hotmail.com> wrote
>
> > The fastest payback for any energy use reduction method is insulation.
> > Loft/attic insulation pays back in a couple of years for example and blown
> > in cavity wall insulation is up there somewhere too.
>
> More attic insulation was going to be next calculation.
Current guidance in the UK is to use 10-12" of glassfibre or rockwool mat (or
equivalent other material) for loft insulation, way more than the 4 inches or so
most people think is adequate from 20-30 year old advice. And it's CHEAP too,
especially if you install it yourself. Just make sure you don't block any air
vents in the eaves in the process..
> But after being disappointed with the windows calc, I didn't have the heart.
> Plus I thought
> maybe my math was off.
It's well known that new double glazed windows have a compatively long payback
period, although the advertising would have you believe otherwise. However if
you need to replace a rotten window, then make it double glazed anyway, as the
incremental cost in that case is low and will pay back in more like 5 years (or
less). Check out local suppliers too rather than rely on higher priced national
companies.
Graham
Kitep
> It's well known that new double glazed windows have a compatively long
> payback
> period, although the advertising would have you believe otherwise. However
> if
> you need to replace a rotten window, then make it double glazed anyway, as
> the
> incremental cost in that case is low and will pay back in more like 5
> years (or
> less). Check out local suppliers too rather than rely on higher priced
> national
> companies.
Guess I can always hope some kid puts a baseball through my window <fingers
crossed> :)
Eeyore
Kitep wrote:
> "Eeyore" wrote:
>
> > It's well known that new double glazed windows have a compatively long
> > payback period, although the advertising would have you believe otherwise.
> However
> > if you need to replace a rotten window, then make it double glazed anyway,
> as
> > the incremental cost in that case is low and will pay back in more like 5
> > years (or less). Check out local suppliers too rather than rely on higher
> priced
> > national companies.
>
> Guess I can always hope some kid puts a baseball through my window <fingers
> crossed> :)
Good Luck with it anyway whichever route you take.
One other nice thing about double glazing is they you're relatively unlikely to
get condensation (and possibly resultant mould) from condensation on the window
pane.
Also look to fix any excessive draughts you may have. Note that it's important
that your home needs some air circulation (so don't make it 'hermetically
sealed') or humidity will increase to unpleasant levels which may then (read
WILL) also lead to internal condensation.
Graham
Paul M. Eldridge
Hi Kitep,
I've only skimmed through your calculations, but there appear to be
two key factors you've overlooked. One is the operating efficiency of
your gas furnace or boiler; to account for this, multiple the number
of BTUs per CCF by the AFUE rating of your heating system -- if, for
example, your furnace's AFUE is 80 per cent say, then your true cost
per MM BTUs is 1.25 times higher than what is shown here. In
addition, your cooling costs per BTU are likely to be very different
from those of your heating, so they really should be treated
separately.
Secondly, your simple payback assumes no increase in natural gas
prices over the next ten to fifteen years and it's safe to say the
cost of natural gas (and, likewise, the electricity you use to run
your CAC) will significantly outpace the rate of inflation going
forward; in fact, the Nymex Henry Hub futures price has more than
doubled in just the past eight months alone.
Other potential considerations include 1) increased comfort (low-e
coatings do make a noticeable difference), 2) reduced uv fading (helps
minimize the damage done to furniture, drapery, rugs, etc.), 3)
reduced sweating and related mould and mildew problems (aluminum clad
frames can be somewhat problematic in this regard) and 4) lower air
infiltration as you seal any gaps and leaks that exist with respect to
your current windows.
You might also ask yourself if these new windows will add to your
home's curb appeal and make the interior spaces more attractive.
Lastly, new windows and doors can increase your home's resale value
and this could be an important consideration should you decide to sell
at some future point.
Cheers,
Paul
Kitep
"Paul M. Eldridge" <paul.e...@ns.sympatico.ca> wrote in message
> I've only skimmed through your calculations, but there appear to be
> two key factors you've overlooked. One is the operating efficiency of
> your gas furnace or boiler; to account for this, multiple the number
> of BTUs per CCF by the AFUE rating of your heating system -- if, for
> example, your furnace's AFUE is 80 per cent say, then your true cost
> per MM BTUs is 1.25 times higher than what is shown here. In
> addition, your cooling costs per BTU are likely to be very different
> from those of your heating, so they really should be treated
> separately.
Good catch. I'm not sure what my natural gas furnace operates at, but since
it vents through the wall instead of through the roof, by code it must be
93% or better (I think).
So 93% -> 1.075 multiplier, and my $9.25/year should really be $9.94/year,
payback time = 14.5 years. That's a year better than before, but still not
enough to make it worthwhile.
> Secondly, your simple payback assumes no increase in natural gas
> prices over the next ten to fifteen years and it's safe to say the
> cost of natural gas (and, likewise, the electricity you use to run
> your CAC) will significantly outpace the rate of inflation going
> forward; in fact, the Nymex Henry Hub futures price has more than
> doubled in just the past eight months alone.
True enough. I also didn't put in the opportunity costs. I think like PV,
it's not worth it yet, but maybe later as energy costs keep increasing --
and I assume window technology gets better as well, the longer I wait.
> Other potential considerations include 1) increased comfort (low-e
> coatings do make a noticeable difference),
I wish I knew a way to test this. My windows definately feel cold in the
winter. I've even referred to them as "cold radiators". I know with walls,
that if an outside wall feels colder than an inside wall, it means you don't
have enough insulation. I was hoping the same was true with windows, ie
that double or triple glaze, low-e, argon filled, would stop them from
feeling cold.
> You might also ask yourself if these new windows will add to your
> home's curb appeal and make the interior spaces more attractive.
My windows look fine. However they have a built in screen that rolls up
into the frame when closed, and on most of windows this doesn't work too
well. It'd be a resell point, but since I don't open my windows very often
it doesn't bother me that much.
J. Clarke
Won't help--they'll just replace the broken glass, not the entire
window, unless it's made in such a way that the glass is not
field-replaceable.
--
--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)
Ecnerwal
"Kitep" <fake_address@no_where.com> wrote:
> More attic insulation was going to be next calculation. But after being
> disappointed with the windows calc, I didn't have the heart. Plus I thought
> maybe my math was off.
In point of fact, it's good to run a (necessarily ballpark) calculation
on the whole house, broken down into components. That lets you see where
the heat and $ are going, which makes it easier to figure out what it
will make sense to spend money on. Since you are already in the as-built
phase, you can also see if the ballpark estimate lines up with actual
use, and adjust the estimate to be more accurate. A spreadsheet works
well for this sort of thing. Use fields and calculations so that you can
enter all the things you might change, such as furnace efficiency.
RESFEN from LBNL is another approach to this analysis, though I find
myself unhappy with some of its built-in assumptions. But it does try to
calculate the effect of sun on the windows, which the simplistic
approach based only on R-values and degree days does not. That can make
quite a difference in some situations.
http://windows.lbl.gov/software/resfen/resfen.html
As for your original question - it almost never makes pure heating
payback sense to replace windows in good condition, which is why buying
good ones when you need to buy them anyway is important. Caulking the
old ones does make sense, and replacing the old ones because you are
dissatisfied with them aesthetically (or they are not in good condition)
does make sense, if you can afford that for that reason.
While your calculations don't include rising fuel prices, they also
don't include (as hard-eyed accountants do) the value of your $144
invested and providing you income, which can make your payback look even
worse.
One figure that is hard to track down (though you might get an insight
if your utility or an efficiency organization offers a blower door
test) that will make a difference is air infiltration, or ventilation
rate. When cold air leaks in and hot air leaks out, that costs you
additional heating $. You need some air exchange, but old houses often
have far more than is advisable, and an "overly tight" house can save a
lot of heat $ by doing its air exchange through an air-air heat
exchanger.
In "British" units the British mostly don't use anymore:
Figure volume of house (conditioned space) in cubic feet.
Divide by 13.9 to get pounds of air.
Multiply by the estimated or measured air changes per hour. Start with
1.0 for a typical older house if estimating.
Multiply by 0.24 (the specific heat of air).
This gives the BTU/HR*DegF
If going by degree days (HDD and CDD) multiply by 24 (day) and the
appropriate degree day figure to get yearly figures.
Add that to the radiant heat loss (simpler calculation you already
grasp, if you can add up square feet of similar R-values) and see if it
tallies with your utility bill, though domestic hot water,
refrigeration, and lights will all have an impact, depending on how your
energy sources are distributed in supplying your needs.
Replacing one window does not make a typical house all that much
tighter. But reducing air infiltration by caulking, sealing, and making
minor repairs is the quickest payback for most houses. Additional
insulation in an unfinished attic space is typically the next - it's a
large area (unlike a window), insulation is cheap (unlike a window), and
adding it to an unfinished attic is low labor cost. Depending on your
water heater and hot water pipes present state of insulation, adding
insulation to those is also a typical good payback project. If your
electricity is expensive, refrigerator replacement can make sense, if
the new fridge is chosen for high efficiency.
--
Cats, coffee, chocolate...vices to live by
daestrom
"Kitep" <fake_address@no_where.com> wrote in message
news:48274214$0$5173$4c36...@roadrunner.com...
Your numbers look about right. I think replacement windows can save you
more if there is a lot of air leakage around/through them. Somewhere I read
that a 'typical' house loses half of its energy via air-exchange. Your
calculations are only looking and conduction through the window. If the old
windows are loose, poorly sealed and drafty, you can see a much higher
savings. So the 'hype' may be exploiting that, but without measurements,
it's just guesswork.
But if you have good windows that seal well and are already double-glazed,
the savings to go to triple glaze is pretty small and usually not worth it.
After the obvious insulation in attic and walls, I'd opt for a 'blower-door'
test and start looking for air infiltration/exfiltration. This *may* lead
you to the windows, but it may lead elsewhere.
daestrom
P.S. Of course cooling costs are the price of electricity, not natural gas.
But between the COP of air-conditioner and the difference in cost of
electric energy versus natural gas, it comes out close.
>
>
daestrom
My low-e windows don't feel as 'cold' to stand next to as the one
sliding-glass door with non-low-e glass (or so it seems to me). The easy
answer for radiant losses is simply thick 'insulated' drapes that you close
at night.
daestrom
mike
You have to borrow money (or forgo investing it) to pay for the windows.
Payback keeps getting worser and worser.
--
Return address is VALID!
Bunch-O-Stuff Forsale Here:
http://mike.liveline.de/sale.html
Kitep
"daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote in message
news:4828bfdb$0$7074$4c36...@roadrunner.com...
>
> Your numbers look about right. I think replacement windows can save you
> more if there is a lot of air leakage around/through them. Somewhere I
> read that a 'typical' house loses half of its energy via air-exchange.
> Your calculations are only looking and conduction through the window. If
> the old windows are loose, poorly sealed and drafty, you can see a much
> higher savings. So the 'hype' may be exploiting that, but without
> measurements, it's just guesswork.
>
> But if you have good windows that seal well and are already double-glazed,
> the savings to go to triple glaze is pretty small and usually not worth
> it.
>
Thanks. That's pretty much the conclusion I've come to. My windows don't
seem to be drafty, but a blower-door test may show otherwise.
> P.S. Of course cooling costs are the price of electricity, not natural
> gas. But between the COP of air-conditioner and the difference in cost of
> electric energy versus natural gas, it comes out close.
>
I wonder why I was thinking my AC ran on NG. I've had to replace the fuse
for it, so I should've known better.
Kitep
>
> My low-e windows don't feel as 'cold' to stand next to as the one
> sliding-glass door with non-low-e glass (or so it seems to me). The easy
> answer for radiant losses is simply thick 'insulated' drapes that you
> close at night.
The difference if feel could simply be caused by the difference in size.
Larger areas of glass are obviously going to feel colder than smaller areas.
Paul M. Eldridge
>> What am I missing? Or have I just fallen for the hype about replacement
>> windows?
>>
>You've left out the time value of money.
>You have to borrow money (or forgo investing it) to pay for the windows.
>Payback keeps getting worser and worser.
Hi Mike,
I'm not so sure. I expect the cost of energy to exceed borrowing
costs and normal rates of return for the foreseeable future. As noted
before, wholesale natural gas prices have more than doubled in the
past year alone (currently $11.32 per MM BTU, up from $5.38 last
August). Given how things stand with regards to natural gas, there's
an excellent chance the payback will be *a lot* sooner than any of us
think.
Cheers,
Paul
Kitep
"Ecnerwal" <Lawren...@SOuthernVERmont.NyET> wrote in message
news:LawrenceSMITH-64C...@news.verizon.net...
> RESFEN from LBNL is another approach to this analysis, though I find
> myself unhappy with some of its built-in assumptions. But it does try to
> calculate the effect of sun on the windows, which the simplistic
> approach based only on R-values and degree days does not. That can make
> quite a difference in some situations.
>
> http://windows.lbl.gov/software/resfen/resfen.html
>
Thanks. I bookmarked the page. I'll figure out air-exchange later this
week.
Paul M. Eldridge
Hi Kitep,
Actually, it's a little more than just that. The glass temperature is
several degrees warmer -- potentially as much as twenty or thirty
degrees warmer in fact.
See:
http://www.askthebuilder.com/B97_Low_E_Glass_-_Savings_and_Comfort.shtml.
I notice a big difference in terms of comfort. With regular windows
you can feel the heat literally being sucked out of your body on a
cold winter's day; far less so in the case of low-e/argon.
Cheers,
Paul
Bruce Harvey
"J. Clarke" <jclarke...@cox.net> wrote in message
news:g09d8...@news3.newsguy.com...
> Kitep wrote:
>> "Eeyore" <rabbitsfriend...@hotmail.com> wrote in message
>> news:482798FC...@hotmail.com...
>>> It's well known that new double glazed windows have a compatively
>>> long payback
>>> period, although the advertising would have you believe otherwise.
>>> However if
>>> you need to replace a rotten window, then make it double glazed
>>> anyway, as the
>>> incremental cost in that case is low and will pay back in more like
>>> 5
>>> years (or
>>> less). Check out local suppliers too rather than rely on higher
>>> priced national
>>> companies.
>>
>> Guess I can always hope some kid puts a baseball through my window
>> <fingers crossed> :)
>
> Won't help--they'll just replace the broken glass, not the entire
> window, unless it's made in such a way that the glass is not
> field-replaceable.
Also in my experience the insurance companies will only replace 'new for
old' or 'like for like'. So if your single glazed window gets broken the
insurer will only pay for a single glaze pane of the same size plus fitting.
At least thats how they work 'down under' where I live. You may be able to
have the upgrade done at the same time but you will still have to pay the
difference. Just a thought.
mike
money.
--
Jeff
I'm sure this varies, but I wonder how people set their thermostats.
It would seem reasonable that if you feel a cold spot that you would
compensate by turning up the heat. Likewise, a blast of solar sunshine
heating may prompt you to adjust the AC down.
>
So, factoring in these little intangible comfort factors may have
outsized influences.
Drapes or some kind of window treatment could have a dramatic effect..
In my old house adding an extra layer of (vinyl) glazing and covering
up the window ACs made a big difference in the heat I was using. I felt
more comfortable at a lower setting.
Jeff
> daestrom
>